In general, the present invention relates to the field of magnetic resonance, in particular the provision of superconducting magnet systems suitable therefor, said magnet systems being destined for producing homogeneous magnetic fields for NMR measurements. However, the applicability of the invention is not restricted to this field.
Magnetic resonance (including nuclear magnetic resonance, abbreviated “NMR”, and including electron paramagnetic resonance, abbreviated “EPR”) is a widespread measurement method, with which chemical compounds may be analyzed. A very homogeneous and temporally constant magnetic field is required in a sample volume to be defined, both in the field of nuclear magnetic resonance spectroscopy (NMR spectroscopy) and in imaging applications (MR imaging), said magnetic field being able to be produced by resistive or superconducting coils or a suitable permanent magnet arrangement.
Magnets which, inter alia, are characterized by a particularly high spatial homogeneity of the magnetic field in the sample space are required for nuclear magnetic resonance (NMR, MRI) applications. Usually, the employed superconducting magnets are equipped with apparatuses rendering it possible to adjust the homogeneity of the field, for example in order to compensate for faults which emerge from unavoidable manufacturing tolerances.
By way of example, “cryoshims”, i.e. superconducting additional coils which, depending on requirements, are charged with a suitably selected current, are examples of such an apparatus.
Another option consists in using suitably shaped structures made of magnetic material (in particular of ferromagnetic material such as e.g. iron, which is magnetized under the influence of the field produced by the superconducting magnet, and may thus be used for field corrections in the sample space).
In some magnet systems, “homogenization structures” are fastened in the room temperature bore of the magnet, as is described, for example, in documents DE 199 22 652 C2 and DE 101 16 505 B4.
This is advantageous in that the homogenization structures can be replaced or modified very quickly, as it is not necessary to discharge or heat the magnet, or even open the housing of the magnet. Since a plurality of iterations are typically required to determine the correct end form of the homogenization structure, this drastically simplifies the homogenization of the magnet.
However, a disadvantageous effect is that the homogenization structure must firstly be subject to good temperature control. Otherwise, unwanted field changes would also arise as a result of a change in the saturation magnetization of the employed material with temperature. Secondly, it is only possible to ensure with difficulty that there is no relative movement between the superconducting magnetic coil and the homogenization structure, for example due to vibrations or thermally caused changes in length of the suspension of the coil. Such relative movement would likewise lead to unwanted changes of the field in the sample space.
In order to avoid these disadvantages, the iron structures are occasionally also fastened directly to the magnetic coil, or at least to the helium tank, in which the magnetic coil is situated, as known from DE 199 22 652 C2, cited at the outset, or from DE 101 16 505 B4.
In order to determine the ideal form of the homogenization structure, the following steps need to be carried out every time in each iteration step:                measuring the magnetic field,        discharging the magnet,        heating the cryostat to room temperature,        opening and ventilating the cryostat,        modifying the homogenization structure,        closing and evacuating the cryostat again,        filling cryogenic liquids and cooling the magnet,        recharging the magnet, and        measuring the magnetic field again.        
Depending on the size of the magnet system, this process may require several weeks and is associated with considerable costs and also risks (e.g. damage to the magnet as a result of thermal cycling).